The present invention relates a vehicle steering system having manual steering mode and automatic steering mode.
Japanese Patent Provisional (Kokai) Publication No. 4(1992)-38266 discloses a conventional steering system as shown in FIG. 10.
The vehicle steering system shown in FIG. 10 has first and second steering mechanism 1 and 3. The first mechanism 1 includes a steering wheel 5, a steering shaft 7, a pinion gear 9, a rack shaft 11, a cylinder unit 13 for power assistance and a hydraulic circuit 15. The second steering mechanism 3 includes a servomotor 17 for driving the steering shaft 7 and an automatic steering control unit 19 for controlling the servo motor 17. When the driver turns the steering wheel 5, the first mechanism 1 steers left and right front wheels 21 and 23, and the hydraulic circuits 15 provides power assistance by supplying the working oil to one or the other of pressure chambers 13a and 13b of the cylinder unit 13. On the other hand, by driving the servomotor, the control unit 19 can rotate the steering shaft 7 and steer the front wheels automatically without the intervention of a driver's manual steering operation on the steering wheel 5.
However, this steering system is unable to hold the steering angle of the first steering mechanism 1 during the automatic steering operation of the second steering mechanism 3 because of its structure arranged to rotate the steering shaft 7 in both of the manual mode and the automatic mode. Therefore, this steering system is liable to arouse unnatural feeling when the steering system is changed from the manual mode to the automatic mode. In the automatic mode, the steering wheel 5 is rotated automatically, and this causes uncomfortable feeling to passengers.